The application claims Paris Convention priority of DE 101 04 054.7 filed Jan. 31, 2001 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a magnet arrangement comprising a superconducting magnet coil system for generating a magnetic field in the direction of a z axis in a working volume disposed on the z axis about z=0, wherein the field of the magnet coil system in the working volume has at least one inhomogeneous contribution Hnxc2x7zn with nxe2x89xa72 whose contribution to the total field strength on the z axis about z=0 varies with the nth power of z and with a field shaping device of magnetic material, which is substantially cylindrically symmetrical with respect to the z axis. The invention also concerns a method for determining the production tolerances of the magnetic field shaping device.
Such an arrangement comprising a superconducting magnet coil system and a magnetic field shaping device is known from U.S. Pat. No. 5,396,208.
Superconducting magnets are used in many fields of application, including, in particular, magnetic resonance methods, wherein one must differentiate between imaging methods (Magnetic Resonance Imaging, MRI) and spectroscopic methods. To obtain good spatial or spectral resolution in such methods, the magnetic field in the sample volume must have good homogeneity. The geometric arrangement of the field-generating magnet coils can optimize the basic homogeneity of the superconducting magnet. Typically, recesses must be provided (so-called notch structures) wherein no wire is wound. This reduces the space for magnet windings which renders the magnet more expensive and increases the stray field.
In an arrangement according to U.S. Pat. No. 5,396,208, an MRI system is designed to be more compact by providing several soft-magnetic rings which replace certain notch structures. These soft-magnetic rings can reduce the size of the superconducting coil system in the direction of the magnet axis such that the system has an opening angle of approximately 90 degrees. A large opening angle for MRI magnet systems is advantageous for claustrophobic patients when the magnet system is used in human medical diagnosis.
In accordance with U.S. Pat. No. 5,396,208, a magnet coil system can be more effectively homogenized using a magnetic field shaping device than with notch structures. Application of this idea to a magnet arrangement for high-resolution resonance spectroscopy instead of an MRI magnet system, produces new problems. To obtain a sufficiently effective field shaping device from magnetic material, at least parts of this field shaping device must typically be mounted close to the working volume of the magnet arrangement. These parts of the field shaping device can also produce, in addition to the desired magnetic field, undesired local distortions of the magnetic field in the working volume of the magnet arrangement due to manufacturing tolerances. This problem is much greater for high-resolution resonance spectroscopic applications than for MRI applications, mainly, for the following two reasons. First of all, the homogeneity requirements on a magnet arrangement for high-resolution resonance spectroscopy are considerably higher than those for an MRI system (typically 2*10xe2x88x924 ppm in a working volume of 0.2 cm3 compared to 5 ppm in a working volume of 34 liters for MRI systems). Secondly, the desired field shaping effect of the magnetic field shaping device of a magnet arrangement for high-resolution resonance spectroscopy is typically obtained with much less magnetic material due to the more compact dimensions as compared to an MRI magnet arrangement. This will produce greatly increased, undesired field distortions if the actual location of some magnetic material of the field shaping device differs from the nominal position by a certain amount. In other words: The efficiency of the magnetic field shaping device is larger with respect to its desired as well as to its undesired effects.
It is the object of the present invention to realize a field shaping device of magnetic material with a suitable geometrical shape in a magnet arrangement for high-resolution resonance spectroscopy such that at least part of the notch structures for field homogenization in the magnet coil system can be omitted and such that the influence of unavoidable manufacturing tolerances of the magnetic field shaping device on the magnetic field shape in the working volume can be sufficiently compensated.
This object is achieved in the invention by using the magnet coil system in an apparatus for high-resolution magnetic resonance spectroscopy, wherein the radial separation of the field shaping device from the z axis is at least partly less than 80 millimeters and compensates for at least 50% of at least one of the inhomogeneous field contributions Hnxc2x7zn of the magnet coil system, and at least one additional coil system is provided which acts as a shim device in the magnet arrangement.
At least one of the inhomogeneous field contributions which occur in the magnet coil system due to omission of the notch structures is compensated by at least 50% by the magnetic field shaping device. The inhomogeneous field contributions have the dependence Hnxc2x7zn with nxe2x89xa72, i.e., their contributions to the overall field strength of the magnet coil system vary along the magnet axis (z axis), about z=0, with the nth power of z.
A particular advantage of a magnet arrangement comprising such a positioned magnetic field shaping device is that field inhomogeneities Hnxc2x7zn, with nxe2x89xa74, can also be compensated for with small amounts of magnetic material. The efficiency of the field shaping device for compensating such field inhomogeneities would be drastically reduced if the field shaping device were completely positioned at a separation larger than 80 millimeters from the magnet axis.
One embodiment of the inventive magnet arrangement is particularly preferred, wherein the actual surface positions of the field shaping device at all locations differ at the most by xcex94P from the calculated surface positions, wherein xcex94P is given by       Δ    P    =      0.2    ·                  ∫                              ∫            C                    ⁢                      ∫                                                            "LeftBracketingBar"                                                            Δ                      C                                        ⁢                                          (                                              r                        ,                        z                        ,                        ϕ                                            )                                                        "RightBracketingBar"                                ·                                                      "LeftBracketingBar"                                                                  J                        C                                            ⁢                                              (                                                  r                          ,                          z                          ,                          ϕ                                                )                                                              "RightBracketingBar"                                                                              (                                                                        r                          2                                                +                                                  z                          2                                                                    )                                                              k                      /                      2                                                                                  ⁢                              ⅆ                V                                                                ∫                              ∫            P                    ⁢                      ∫                                                            "LeftBracketingBar"                                                            ∇                      →                                        ⁢                                          xc3x97                                                                        M                          →                                                ⁢                                                  (                                                      r                            ,                            z                            ,                            ϕ                                                    )                                                                                                      "RightBracketingBar"                                                                      (                                                                  r                        2                                            +                                              z                        2                                                              )                                                        k                    /                    2                                                              ⁢                              ⅆ                V                                                        
with
xcex94c Production tolerance for the maximum (radial or axial) displacement of a volume element dV of the magnet winding in the magnet coil system,
k lowest degree, except for zero, of all those coefficients of the field of the magnet coil system, when expanded in spherical harmonic functions, for whose compensation no shim coil system is provided, wherein the degree characterizes the lower index of the Legendre function Pkm which occurs in the associated spherical harmonic,
Jc current density in the magnet coil system,
{right arrow over (M)} magnetization of the field shaping device,
r radial separation of the volume element dV from the z axis,
xcfx86 azimuthal angle of the volume element dV.   ∫            ∫      C        ⁢          ∫              xe2x80x83            ⁢              …        ⁢                  xe2x80x83                ⁢                  ⅆ          V                ⁢                  xe2x80x83                ⁢        and        ⁢                  xe2x80x83                ⁢                  ∫                                    ∫              P                        ⁢                          ∫                              xe2x80x83                            ⁢                              …                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                                      V                    :                                                                                          
volume integral over the volume of the magnet coil system and the field shaping device.
The above integral over the volume of the magnet coil system is a measure of the field distortions in the working volume of the magnet arrangement which are produced by deviations of the position of the wire windings in the magnet coil system from their nominal positions due to manufacturing tolerances. The corresponding integral over the volume of the magnetic field shaping device is a measure of the field distortions which are produced by manufacturing tolerances in the field shaping device surfaces. If no actual surface of the field shaping device differs from its intended position by more than the amount xcex94P, calculated according to the above formula, it is ensured that, in the working volume, the field shaping device produces substantially only undesired field distortions caused by manufacturing tolerances, which can be compensated for by the additional shim devices. The field distortions of higher order remain smaller than 20% of those field distortions which would be present in the working volume of the magnet arrangement without the magnetic field shaping device, and can therefore be tolerated.
One embodiment of the inventive magnet arrangement is particularly advantageous with which the magnet coil system has active shielding. This active shielding reduces the stray field of the magnet arrangement such that more space for other applications is available in the laboratory.
In a particularly preferred embodiment of the inventive magnet arrangement, the magnet arrangement is provided with passive shielding. The passive shielding has the great advantage over active shielding that it can even increase the field in the working volume.
In a further preferred embodiment of the inventive magnet arrangement, the field shaping device is disposed at least partially radially within the innermost wire winding of the magnet coil system. The efficiency of the field shaping device for compensating the inhomogeneous field contributions Hnxc2x7zn of the magnet coil system is particularly small for close separations from the z-axis.
One embodiment of the inventive magnet arrangement is also advantageous with which the field shaping device is magnetically completely saturated and magnetized only in the axial direction (a direction parallel to the z axis). In this case, calculation of the field produced by the field shaping device is particularly simple and precise.
In two further embodiments, the magnet arrangement is characterized in that the magnetic field of the field shaping device comprises a part H4xc2x7z4 with H4 greater than 0, whose field contribution on the z axis about z=0 varies with the fourth power of z. Moreover, the contribution H6xc2x7z6 to the magnetic field of the field shaping device is substantially zero in these two embodiments. These embodiments have the advantage that, at least part of the typically very complicated notch structures in the magnet coil system which otherwise compensate for the negative field contributions of fourth degree of the magnet coil system, can be omitted due to the positive contribution H4xc2x7z4 of the field shaping device to the overall field of the magnet arrangement. In addition, the negligible contribution of sixth degree from the field shaping device ensures that the total amount of sixth degree contributions to the magnet arrangement is not increased by the field shaping device. This is important since this part of the magnetic field normally determines the size of the volume in which the field of the magnet arrangement has the homogeneity required for high-resolution resonance spectroscopy.
In a particularly preferred embodiment, the field shaping device consists of a ring which is located on an average radius a and extends axially between xe2x88x92z1 und z1, wherein z1 greater than a. This solution is particularly attractive due to the simple geometry of the field shaping device.
In a further advantageous embodiment of the inventive magnet arrangement, the field shaping device comprises two rings which are located on an average radius a and extend axially between z1 and z2 and between xe2x88x92z2 and xe2x88x92z1, wherein 0.42a less than z1 less than 0.46a and a less than z2. Similar to the above-mentioned embodiment, such a field shaping device only produces a small H6xc2x7z6 contribution to the field in the sample volume. The field contribution of order H4xc2x7z4 with H4 greater than 0 is considerably larger than in the previous embodiment.
In a further advantageous embodiment of the inventive magnet arrangement, the field shaping device comprises components of soft iron. Advantageously, soft iron has large permeability and high saturation induction. These properties provide the field shaping device with high magnetization such that even small amounts of material produce high field efficiency.
In an additional advantageous embodiment of the inventive magnet arrangement, parts of the field shaping device are subjected to surface treatment, in particular, galvanization. This surface treatment offers optimum protection from corrosion which is absolutely necessary, in particular, for components made of soft iron.
In one particularly preferred embodiment of the inventive magnet arrangement, the field shaping device consists of one single element of magnetic material. This is the simplest possible embodiment for the field shaping device with regard to production and assembly.
In another advantageous embodiment of the inventive magnet arrangement, the field shaping device comprises several elements of magnetic material. This offers more freedom for optimizing the field shaping device.
In a further advantageous embodiment of the inventive magnet arrangement, the field shaping device comprises magnetic sheets which are disposed on a carrier device. The efficiency of the magnetic material close to the z axis is sufficiently large that little material is required for producing the desired field shape. Sheets therefore offer an ideal solution, in particular since they have a substantially non-varying thickness.
Two further advantageous embodiments of the inventive magnet arrangement are characterized in that existing components of the magnet arrangement can be utilized for the field shaping device. In the first embodiment, the field shaping device comprises components which are part of a coil form of the magnet coil system. Magnetic material can e.g. be evaporated onto a carrier device.
In the second embodiment, the field shaping device comprises components which are part of the cryostat in which the magnet coil system is accommodated. Both embodiments have the advantage that no additional parts are required for the field shaping device, thereby saving space e.g. for magnet windings.
In a further embodiment, the magnet arrangement is characterized in that the field shaping device comprises components which are disposed in a region of the magnet arrangement which is at room temperature. These components are easily accessible during operation and can be modified without heating up the magnet coil system.
In a particularly preferred embodiment of the inventive magnet arrangement, the field shaping device comprises cooled components, in particular such that these have the temperature of the liquid helium bath which cools the magnet coil system. The low temperature advantageously improves the magnetic properties of the magnetic material, i.e., larger magnetization for a given external field. When the temperature is stable, fluctuations in the magnetization are also suppressed which guarantees improved temporal stability of the homogeneity of the magnet arrangement.
A method for determining the production tolerances of the magnetic field shaping device also lies within the scope of the present invention and is characterized in that the value xcex94P, which determines the maximum deviation of the actual surfaces of the field shaping device at any location from the ideal surface locations, is calculated, wherein xcex94P is given by       Δ    P    =      0.2    ·                  ∫                              ∫            C                    ⁢                      ∫                                                            "LeftBracketingBar"                                                            Δ                      C                                        ⁢                                          (                                              r                        ,                        z                        ,                        ϕ                                            )                                                        "RightBracketingBar"                                ·                                                      "LeftBracketingBar"                                                                  J                        C                                            ⁢                                              (                                                  r                          ,                          z                          ,                          ϕ                                                )                                                              "RightBracketingBar"                                                                              (                                                                        r                          2                                                +                                                  z                          2                                                                    )                                                              k                      /                      2                                                                                  ⁢                              ⅆ                V                                                                ∫                              ∫            P                    ⁢                      ∫                                                            "LeftBracketingBar"                                                            ∇                      →                                        ⁢                                          xc3x97                                                                        M                          →                                                ⁢                                                  (                                                      r                            ,                            z                            ,                            ϕ                                                    )                                                                                                      "RightBracketingBar"                                                                      (                                                                  r                        2                                            +                                              z                        2                                                              )                                                        k                    /                    2                                                              ⁢                              ⅆ                V                                                        
with
xcex94c production tolerance for the maximum (radial or axial) displacement of a volume element dV of the magnet winding in the magnet coil system,
k the lowest degree, except for zero, of all coefficients of the field of the magnet coil system in the expansion according to spherical harmonic functions which is not compensated for by a shim coil, wherein the degree characterizes the lower index of the Legendre function Pkm occurring in the associated spherical harmonic,
Jc current density in the magnet coil system,
{right arrow over (M)} magnetization of the field shaping device,
r radial separation between volume element dV and z-axis,
xcfx86 azimuthal angle of the volume element dV.   ∫            ∫      C        ⁢          ∫              xe2x80x83            ⁢              …        ⁢                  xe2x80x83                ⁢                  ⅆ          V                ⁢                  xe2x80x83                ⁢        and        ⁢                  xe2x80x83                ⁢                  ∫                                    ∫              P                        ⁢                          ∫                              xe2x80x83                            ⁢                              …                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                                      V                    :                                                                                          
volume integral over the volume of the magnet coil system or the field shaping device.
Keeping the determined production tolerances for the magnetic field shaping device ensures that the field shaping device produces substantially only undesired field distortions in the working volume, which can be compensated for by the shim devices, wherein the field distortions of higher order remain smaller than 20% of those field distortions which would be present in the working volume of the magnet arrangement without the magnetic field shaping device.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and further explained by means of embodiments.